How can one mock this?

config.clj

(ns config)

; to be populated on application start before `service` is required.
; how can this def be mocked?
(def settings {})

service.clj

(ns service
  (:require [config :refer [settings]]))

(println "initializing namespace")

(def customer-realm
  (get-in settings [:auth-rocket :realms :customer]))

service_test.clj

(ns service-test
  (:require [service :as service]))

(defn- reload-with-settings [settings func]
  (with-redefs [clojurewerkz.gizmo.config/settings settings]
    (require 'user-management.auth-rocket-test :reload)
    (func)))

(testing "the service"
  ; this is the magic part, how do you do this?
  (update-the-settings-and-reload-service-with-config {:auth-rocket {:realms {:customer "abc"}}})
  (is (= "abc" auth/customer-realm)))